Note: Descriptions are shown in the official language in which they were submitted.
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1 PHD 78097
The invention relates to a device for deter-
mining the internal structure of a flat examination zone
of a body, comprising a radiation source which emits a
primary radiation beam which extends in the plane of the
examination zone and which has a small cross-section,
said source being displaceable in a direction which
extends in the plane, transversely of the direction of
the primary radiation beam, and also being rotatable
around an axis extending perpendicularly to said plane,
comprising a detector device for generating measuring
signals by the detection of the radiation emerging from
the body, and also comprising a computer and display
device for calculating, from the measuring signals, and
for displaying the internal structure.
A device of this kind :is already known from
German Auslegeschrift 19 41 433 G.N. Hounsfield - pub-
lished February 26, 1970. This device comprises an ~-
ray source or gamma radiation source which emits a prim-
ary radiation beam having a small cross-section which
irradiates a flat examination zone of a body in differ-
ent directions, situated in ~he plane, each time along
radiation paths which are situated in parallel in one
direction. The primary radiation beam is detected by
means of a detector for generating transmission signals.
Using a computer, the distribution of the absorption
or transmission coefficients of the irradiated slice
~ .
-
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2 P~ID 7~o97
O:r the bocly is calculated frorn the transmission sig-
nals.
To this end, the irradiated slice of the body
is imagined to be divided into a matrix of elements, an
absorption or transmission coefficient being displayed
in each element.
However, in the body the primary radiation
beam is substantially attenuated along its radiation
paths by absorption and scattering. The ratio of the in-
tensity before and after the passage through the body,therefore, is comparatively large Because the accuracy
of the reconstruction, however, is dependent of the in-
tensity of the prirnary radiation beam after the passage
through the body, the intensity of radiation must be
comparatively high prior to the passage through the
body, so that a high radiatlon :load for the body occurs.
The invention has for its objcct to provide
a device which enables determination of the internal
structure of a body with a comparatively low radiation
load for the body.
To this end, a device in accordance with the
invention is characteri2ed iIl that the detector device
is not struck by the primary radiation beam and is
situated at least on one side of the plane in order to
de-tect scat-tered radiation gellerated by a primary ra-
diation beam along its beam path so that a scatter sig-
nal. is obtained which is associated with the b.eam path,
said detector device at least partly enclosing the body,
the computer calculating a distribution of the scatter
coefficients in the irradiated plane on the basis of the
scatter signals obtained.
Primary beams of X-rays or gamrna rays in the
energy range of from one hundred to a few hundreds of
kilo-electronvolts (keV) are attenuated by incoherent
scatter:ing (Compton scattering) during irradiation of
a body. ~rhen a human body is e~amined, the photoelectric
absorp-tion occu:rring in the body can be neglected as
from radiation energies of approximatel~ 150 keV. In the
3 P~ 78O97
field o-~ examination o~ materials, the photoelectric
absorption can be neglected as from radiation energies
of some hundreds of keV, depending on the nature o~ the
material t~ be examined~
On the basis of the comparatively high atte-
nuation encountered by the primary radiation beam during
its passage through a body, the ratio of the intensity
before and after the passage through the body is very
large~ whilst the ratio of scat-tered and incident radia-
1n tion is substantially larger. The relative inaccuracies
(~ It/It and ~ IS/Is) with which the intensity of the
radiation behind the body (It) and the intensity (I )
of the scat-tered radiation, respectively, is measured
and which influence the quality o~ the reconstructed
structurb distribution are inversely prop~rtional to
the root of the in-tensi-ty of the transmitted and
scattered radiation, respectively, Thus, the relative
inaccuracy of the intensity of the transmitted radia-
tion is much higher than that of the intensity of the
scattered radiation. By measurement of the scattered
radiation it can be achieved that the reconstruction
of a scatter coefficient distribution can be performed
with the same accuracy as the reconstruction of an ab-
sorption coef~icient distribution obtained from measure-
ments o~ the -transmitted radiation, but already with a
much lower intensity of the incident primary radiation
beam~ so that the radiation load for the body is sub-
stantially reduced.
The scatter signals are then measured by mea-
surement of an as large as possible part of the scatter-
ed radiation generated by the primary radiation beam
along a beam path. For this it is necessary that the
detector device envelops the body to be examined as
completely as possible. From the scattered radiation
signals obtained, the distribution can be calculated
of scatter coefficients which represent the attenuation
of the radiation on the basis of -the scattering at ele--
ments of a matrix ~hich is permanently associated with
- . - .. . .. .. .
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4 PHD 78O97
the body. The i.ndividl1a.l scatter coefficients at the --
~matrix elements can then be converted into correspond-
ing grey values for the display of i.ndividual slice
images and can be made visible, for example, on an
image matrix (monitor) or on a printer.
In a further attractive embodiment of the in- ;
vention, the detector device comprises two identical,~
hollow-cylinder detectors which together substantially
completely envelop the body and which are arranged at
a distance from each o$her in a mirror-inverted manner~ ;
with respect to the plane, cylinder axes of the cylind~er
detectors being coincident and extendin~g transversely ofc
the plane.
By means of such a measuri.ng arrangement it
is achieved that the scattered radiation which is emi-tted
at a larger angle with respect to the radiation direc-
tion of the primary radiation beam is also measured, so
.~ that the scattered radia-tion generated is substantially
: completely detected and the accuracy of the reco:nstruct-
ed scatter coefficient distribution is improved,
The output signals generated by means of the
two hollow-cylinder detectors, bcing a measure for the
scattered radiation generated by the primary radia-tion
; beam~ are added in order to determine a scatter signal.
To this end, the hollow cylinder detectors are c~nnected
: to an adding stage for the adding of their output sig-
nals.
In a further embodiment of the invention, the
hollow-cylinder detectors form an integral unlt and are
rotatable about their cylinder axis and displaceable in
a direction perpendicular to the cylinder axis, the
cylinder detectors being provided with apertures for
passage of the primary radiation beam which are situa-t-
ed in the plane and radially opposite each other. A cy-
lind.èr detector of this kind also enables measurementof sca-ttered radiation occ~rri.ng in the plane, so -that
only a ~ery small par-t of the total scattered radiati.on
generated is not detected. In the extreme case, the ....
., ., . . . . . . .. . .. . .... . ... . . . . . . .. . . . ... , . . _ ... .. .. .. .. . .
:
~ P~ID 78O97
..
scattered r~diation emitted in the direction of the ...;
cyl.inder axis can also be measured if one of the outer
end faces of the hollow-cylincler detec-tors is covered
with detector plates.
In an attractive fur-ther embodimen-t of a device
in accordance with the invention, moreover, a detector
for detecting the primary radiation beam is arranged in
the plane. Besides the measurement of the scattered ra-
diation generated by the primary radiation beam along
its beam path, the radiation of the primary ra.diation
beam which is transmitted by the body is also measured -
for generating transmission signals. It is thus ach.ieved
that also in an energy range of the radiation of the
primary radiation beam in which the photoelectric ab~
sorption can no longer be neglected, a distribution
of the scatter coeffi.cient can be reconstructed by a
combination of transmission sig~nals and scatl:er sig~lals
while the radiation load for the body i 5 reduced.
~n ernbodimen-t of the d.evice in accordance
with the invention will be described in de-tail herein-
after with reference to the accompanying di.agrammatic
drawing.
Figure 1 shows a detector device for measur-
ing the scattered radia-tion and a further detector for
measuring the transmitted radiationg~.
Figure 2 shows a device in accordance with
the invention with the detector device and the further
detector and
~ `igure 3 shows a process diagram of a method
of determining a scatter coefficient dis-tribution.
Figure 1 shows an X-ray source 1 which emi-ts
a thin primary radiation beam 3 which is collimated hy
an apertlJre 2 and which extends in the X-Z plane of a
carthesian system of coordinates X~Z. The primary ra-
35 diation beam 3 has a small angle of aperture (not shown). ~ i
The beam path l(r, 9) of the radiati.on is defined by the
distance r from the origin 6 of the system of coordinates
and the angle ~ enolosed by the path and the X-axls, .~
~ ., , . " ,, . . ", . . .......... ..... . .
LQ66~
... . . , . .. .. ..... . ~ . . . . .. .. ..
~ PI~ 78O97
~ 't
viewed from the source 1. A body 5 to be examined which
extends in a direction perpendicu]ar to the plane ~
(y-a~is) is irradiated by the primary radiatiGn beam 3 in
different directions ~ situated in the plane 4, in each
direction ~ in parallel beam paths l(r, ~), for which
each time a different distance r is adjusted. The position
of the primary radiation beam 3 is changed by displace~
ment or rotation of the X-ray source 1 in the plane ~ (to
be described with reference to Figure 2), so that a flat
examination zone of the body 5, which may be f`or example,
a hum~n body, can be scanned. The position of the body
5 or the hollow-cylinder detectors 9 and 9' is not chang-
ed. ~-
The scattered radiation emerging from -the body
5 and generated by the primary radiation bearn 3 is de-
tected by a detector device ~ which consists of two
- hollow-cylinder detectors 9 and 9' wh:ich have the same
shape and which are arranged in a mirror-inverted man-
ner on both sides of the plane 4 so that their cylinder
a~es 10 and 10' (y axis) extend perpendicularly through
the plane 4 and are coincident. Both hollow-cylinder de-
tectors 9 and 9' are removed from each other only so far
that they just fail to impede the primary radiation
beam 3 passing therebet~een. The two cylinder detectors
9 and 9~ measure the major part of the scattered radia-
tion each time generated by the primary radiation beam
3 in a beam path l(r, Q). The individual hollow-cylinder
- detectors 9 and 9' produce output signals A(r, ~) and
A'(r, ~) which correspond to the measured scattered
radiation and which are added in order to form a scatter
signal S(r, ~ igure 2). There is also provided a de-
tactor ~ which is arranged in the plane ~ and which
measured -the transmitted radiation of the primary ra-
diation beam 3 in order to generate transmission signals
T(r, ~).
~ rom the scatter signals ~(r, ~) obtained for
all beam paths l(r, ~) alone, or in combination with
the transmission signals T(r9 ~), a distribution of
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... . . .. . . . . .. .. .. . .. . . . . ...
7 PHD 78097
scatter coefficients at elements of a matrlx in the
plane l~ which describes the internal structure of the
irradiated body 5 c`an thell be reconstructed. This will
be elaborated hereinafter,
Obviously~ the X-ray source 1 may also be re-
placed by a gamma radiation source or by a corpuscular
radia-tion source, for example, a neutron radiation
source. The detector device 7 is then chosen according-
ly or its spectral sensitivity is adapted to the radia-
tion of the relevant radiation source. The hollow-cy-
linder detectors 9 and 9' may be constructed, for exam-
ple, as ioni~ation detectors which consist each time of
two concentric hollow cylinders 11 and 11' which are
arranged one inside the other and wherebetween a
pressuri~ed ionizable gas is provided and wherebetween
an electric voltage is applied.
The two hollow cylinders 9 and 9~ may a~ter- ;
natively form an integral unit in order to increase the
measurable scattered radiation~ Thus, the scattered ra-
~20 diation which emerges from -the bodv aIld which extends
in the plane 4 is also measured. The primary radiation
beam 3 then pass~s through an entrance and an exit win-
dow (not shown), the passage o~ the radiation beam 3
(obviously) not being impeded by the scattered radiation
detector 7. During displacement or rotation of -the X-ray
source 1 or the primary radia-tion beam 3, the integral
cylinder detector 7 should then be displaced and rotated
accordingly, so that the primary radiation beam 3 can
always pass through an entrance window and an exi-t win-
dow. The body 5 remains stationary.
~ n order to increase the part of the scatteredradiation to be measured each time along a beam path
l(r, 0), one of -the end face of the hollow-cylinder de
tectors 9 and 9', which are remote from the plane 4 and
which are situated o~ the outer side~ can be covered
with a detector pla-te, so that scattered radiation
emitted in the direc-tion of the cylinder axes 10 and 10'
lS also detectedq
-- .
~3~
8 PHD 78097
Figure 2 shows a device in accordance with the
invention l~hich comprises two hollow-cylinder detectors
9 and 9' which enclose a body 5 arranged on a table 12
which is displaceable in three coordinate directions
x, y, z. The radiation source 1 and the detector 8 are
arranged one opposite the other on a displaceable sup-
port 13 which can be intermittently or continuously
disp~aced in a d:irection in the plane 4 (x-y plane) by
: means of a gearwheel 15 which is driven by a motor 14
and a rack 16. The body 5 can thus be irradiated along
a large number oI` parallel beam paths l(r, ~), situated
in the plane 4), in an angular position ~.
The support 13 is arranged, by means of bear- i
ings 17, between two bearing rails 18 and 18' which are
rigidly connected to a supporting frame 19 which has a
cireular circumferellce and which is guided on guide rol.-
lers 20~ The guide rollers 20 are supported by a sta-
tionary frame 200 (only diagrammatically shown), In
order to change the direct:ion e of the radiation beam
3 in the plane 4 (see Figure 1), the supporting frame
19 is rotated, by means of further motor 21 whose shaft
; ~ accommod.ates a further gearwheel 22, around a:n a~is
which extends perpendicularly to the plane ~ (x-direc-
~ ~ tion) a~d which may be, for example, -the cylinder axis
: 25 10 and 1~'. To this end, the further gearwheel 22 engages
a gear ring 23 (only partly shown) which is situated on
the circumferenee of the supporting ~`rame 19. The hollow-
eylinder detectors 9 and 9' are not moved (relevant hol-
ders are not shown for the sa~e of clarity of the draw-
ing).
: The deteetor 8 which measu~res the transmitted
radiation of the primar~ radia-tion beam 3 is eleetri-
eally eonneeted to an amplifier 24 whieh is eleetrieally
eonneeted to a logarithmation unit 25 for amplification
and logarithmation of the transmission signals Ttr, ~)
generated by tb.e detector 8. The logarithmation unit 25
has eonneeted to it an analog-to-digital eonverter 26
... and theret~ an electronic memory 27 is conneeted for.the
., . , . . , .. . . .. , . .. , .. , .. . . , . -- .. . .. .... ... ....... . .. ......... . . .....
.
~iL3~
9 Pl-~ 78O97
storage of the transmission signals T(r, O) in digital
form
There is also provided an adding stage 28 which
adds the ana:Log output signals A(r, O) and A'(r, ~) on
the outputs of the hollow cylinder detectors 9 and 9'
in order to form a scatter signal S(r, ~). The scatter
signals (r, O) is amplified by an amplifier 29 and is
digitized by the analog-to-digital con~erter 26. The
sca-tter signals S(r, ~) thus converted are then also
stored in the electronic memory 27. ~ computer 3O which
is connected to the electronic memory 27 determines~ on
; the basis of the scatter signals S(r, O) alone or in com- `
bination with the transmission signals T(R, ~) obtained,
a distribution of the scatter coefficient ~(m n) at
elements of a matrix situ~ated in the plane 4 and hav-
ing n rows and m columns of elements, which distribution
of ~(m.n) is displayed on a display screen 31.
The computer 3O furthermore comprises a control
` system for control of the motors 1~and 21 for performing
a displacement or rotation of the primary radiation bearn
3 in the plane 4. The predetermined distance r or the
angle ~ of each beam path l(r, O) is s-tored in the elec-
tronic memory 27 in order to be aYailable for the recon-
struction at a later stage. The computer 3O is further-
more connected to an X-ray generator 32 for control of
the ~-ray tube 1.
Fig. 3 shows a process diagram of a method
for determining a distribution o~ scatter coefficients
~rom scatter signals S(r, O), or sca-tter signals
S(r, O) and transmission signals T(r, O), measured by
means of the deYice in accordance with the invention.
l~hen the energy of the primary radiation beam
3 is in the range of from approximately one hundred to
a few hundreds of keV, the photoelectric absorption in
35 the body 5 can be ignoredO This can be done for radia- ~ ;
tion energies of from approximately 150 keV in human
bodies, while in the case of examination of materials
radiation energies of a few hundreds of l~eV are neces-
.. . .. . .. . . . .. . ... .. .. .. ... . . .. .. ......... . .. . ...... . . . .. ... . ...... . . ...........
~.3~
.. ... . . . .. .... . . . .. . . ...
P~D 7~O97
sary.
In a f`irst step (field 33) of the method, the
hollow-cylinder detectors 9 and 9~ are used for record-
ing for all beam paths (l(r, ~) the output .signals
A~r ~) and ~'(r, ~ hich represen-t the intensity of
the scattered radia-tion along a beam path l(r, ~) of the
primary radiation beam 3 and which are each time added
for determining scattered signals S(r, ~).
Use is made of a previously selected distri-
bution of scatter coefficients6(m, n) on the elements ofthe ma-trix situated in the plane ~ (see Figure 1) which
approximates the internal structure of the body 5. This
distribution may comprise, for example, equally large
scatter coefficients ~(m, n) which are also stored in
the memory 27, All beam paths l(r, ~) are successively
fetched (field 35, step t~.To of the method) for calau-
latillg ~catter data S(r~ ~) :Por the indi~idual beam
paths l(r, ~), the angular variation ~ being effecti~re-
ly reali~ed in steps of appro~imatcly 11o. The coordi-
nates r ancl ~ of the beam patlls l(r, ~ are alreadystored in the memory 27 for this purpose. Dur:ing a
next step, the scatter da-ta S(r, Q) are calculated
in the computer 3O with the predetermined distribution
~(m, n) of the scatter coefficients (field 36). The
~5 scatter data S(r, ~) are determined by using -the for-
mu~e:
S`(r~ ~) = So ~ ~.KF.dl (1)
The equation (1) provides a line integral of -the scatter
coefficient ~ over the beam path 1, 1 being l(r, ~).
The scatter coefficient ~ is multiplied by a factor K
which describes the activation power of the hollow-
cylinder detectors 9 and 9'. This factor K can be de-
termined with high accuracy by the hollow-cylinder de--
tectors 9 ~md 9' by irradiating a test object along all
beam patlls l(r, ~) by means of the primary radiation
bean1 3 and by measuring the scatter radiation (known)
... . .. . .
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11 P1~ 78O97
then occurring.
Furthermore 9 the scatter coefflcient 6- is
multiplied by an at-tenuation factor F which describes
the attenuatlon of -the primary radiation beam 3 alon~
the beam path l(r, ~) as far as an important element
of the matrix If the important element of the matrix
on the path l(r, ~) is denoted by the reference i~
the attenuation f`actor F for the element; i; can be
described as follows when the photoelectric absorption
is neglected:
Fi = exp ~ (- C~(j) . ~) (j) ) (2)
j=l . ' '
Therein, the scatter coe~ficients ~ (j) of all ele-
ments ~; situated in the beam pa-th l(r, ~) before the
relevant element i are summed, each time weighted
with the expansi.on w(j) o:~ the elements i in the direc-
tion of the beam path l(r~ ~). T.he expansion ~(j) is then
small in comparison with the characteristic changes of
the i.nternal structure of the body 5. The equation (1)
furthermore comprises a factor So which indlcates the
knowll or predetermined output intensity of the X-ray
source 1. The line integral (formule 1~ can be con-
; verted into a sum, after which the scatter values
S(r, ~) can be calculated in a numerical manner for
al]. beam paths l(r, ~), taking into account the formule
(2).
The next step (.field 37) of the method com-
prises the comparison of the scatter values S(r, ~)
and the sc~tter signals S(r, ~) for each beam path
l(r, ~ orrection data C~, where ~ = S(r, ~)/S(r, ~), ;
are calculated in a next step of the method ~ield 38)
to correct all scatter coefficients ~ of the associat~
ed beam path l(r, ~ in accordance with
~ = 6 ^ ~ (3~
The corrected scatter coefficients or are stored in the
memory 27. . -~
.. . . . .. .......... . ........ ........... . ..
-
~3fl~
.... ... . .. ....
12 Pl~ 78O97
.
~ fter de-termination and processi.ng (test in
fielcl 39) of the correction data for all beam paths
(r, ~), it .sis tested (field l~o) whether the difference
( ~ ) bet~een the each time predetermined scatter coeffi-
cients ~ and the corrected scatter coefficients 6-
is small enough to allo~ termination of the iteration
process. If` the deviation between the coefficients
.~~ and ~~is too large (¦ 1 - CX/~ S~ ~ being a
small numb~r to be chosen, the steps in the fields
`10 34, 35, 36, 37, 38, 39 are executed again, the cal
culated scatter coefficients G' then being considered
as being predetermined. If the routine loop 4OO has
been comploted an adequate number of times,
/ 1 - ~ / 5 S and the scatter coefficiellts are suf-
ficiently accurate.
Subsequently (field 41), the scatter coef-
ficient distrihution i.s displayed, for example, on the
monitor 31.
If the photoelectric absorption in the body
~ cannot be neglected, for example, in the case of a
radiation ensrgy of the primary xadiation beam 3 in
the range of some tens of kilo-electronvol-ts, the
att0nua-tion factor Fi is described by the below formule
(4~ instead of the formule (2):
i~1
~i = exp ~ ( - (6~ K(j) ) ~J (j) ) (4)
The addend k(j).therein indicates the part of the photo-
electric absorption in the attenuation of the primary
radiation beam 3. For calculating the scatter data
S(r, ~) (field 36), where also the attenuation of the
radiation scattered in the body 5 is -ta.ken into account,
a sum /1l(j) of the scal;ter coefficient 6~ (j) ~nd the
absorption coeffi.cient k(j) must each time be known for
a matrix e].ement ~. The sum /u(j), indicating th.e atte-
nua-tion coefficient of a matrix element j, can be cal-
culated in know-n manner by reconstruction o.t -the atte-
. nuation coefficient distribution (see ~.~. Brooks and
, . . . ..... ...... . . . . ... .... ....... . ............... . .
~3~0~
13 PHD 7SO97
G. di Ghiro, Phys. Med. Biol. 1976, Vol. 21 J No.
5,689-732). However, during the measurement of the
scatter signals S(r, O), also the transmission signals
T(r, ~) must then also be measured by measurement of the
radia-tion of the primary radiation beam 3 transmitted
by the body 5.
